The invention relates to a reflection grating and to an optical multiplexer or demultiplexer provided with such a grating.
Such a reflection grating is suitable for use in optical telecommunication systems equipped with light-conducting fibers. A multiplexer or demultiplexer provided with a reflection grating is used on the transmitting or receiving end of a glass fiber communication system in order to combine light beams of different wavelengths into one beam or to split one combined beam into separate beams of different wavelengths.
Alternatively, a transmission grating, such as a so-called Bragg deflector, may be employed as a beam splitting element (demultiplexer) or a beam combining element (multiplexer). However, a reflection grating has the advantage that it can be used in a so-called Littrow configuration. In this configuration, the input fiber is disposed in the immediate vicinity of the output fiber.
The article entitled "Grating demultiplexers with printed geodesic lenses in glass" by H. J. Lilienhof et al (Techn. Digest of the 8th ECOC, Cannes, 1982, pages 321-324) describes the use of a reflection grating in an optical demultiplxer. This demultiplexer comprises a planar geodesic lens. A planar optical element may be defined as an optical element comprising a light-conducting layer deposited on a substrate. The layer has a refractive index which differs from that of the substrate and is provided with a structure chosen to achieve a selected optical function. The structure may be a depth profile or a refractive index profile. The refractive index profile can be obtained by ion bombardment or by ion exchange in a salt solution.
A geodesic lens is a planar lens having a depression in a substrate on which a light-conducting layer of constant refractive index has been deposited. In the demultiplexer described in the Lilienhof et al article, the reflection grating is a conventional grating which is arranged against the edge of the substrate of the geodesic lens. The grating lines extend perpendicular to the plane of the thin film conductor.
The Lilienhof et al grating is not integrated with the other components of the optical circuit. Apart from the geodesic lens, the circuit also comprises input and output fibers.
The fact that the reflection grating is not integrated has several disadvantages. For example, during assembly of the optical circuit the reflection grating must be aligned accurately relative to the light-conducting fibers. Further, the components should be adjustable. Moreover, a demultiplexer with a separate grating is more bulky than and less stable than a fully integrated demultiplexer.